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Date: Mon, 13 Sep 2010 14:55:11 +0100
From: Mel Gorman <mel@....ul.ie>
To: Wu Fengguang <fengguang.wu@...el.com>
Cc: "linux-mm@...ck.org" <linux-mm@...ck.org>,
"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
Linux Kernel List <linux-kernel@...r.kernel.org>,
Rik van Riel <riel@...hat.com>,
Johannes Weiner <hannes@...xchg.org>,
Minchan Kim <minchan.kim@...il.com>,
Andrea Arcangeli <aarcange@...hat.com>,
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Dave Chinner <david@...morbit.com>,
Chris Mason <chris.mason@...cle.com>,
Christoph Hellwig <hch@....de>,
Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 09/10] vmscan: Do not writeback filesystem pages in
direct reclaim
On Mon, Sep 13, 2010 at 09:31:56PM +0800, Wu Fengguang wrote:
> Mel,
>
> Sorry for being late, I'm doing pretty much prework these days ;)
>
No worries, I'm all over the place at the moment so cannot lecture on
response times :)
> On Mon, Sep 06, 2010 at 06:47:32PM +0800, Mel Gorman wrote:
> > When memory is under enough pressure, a process may enter direct
> > reclaim to free pages in the same manner kswapd does. If a dirty page is
> > encountered during the scan, this page is written to backing storage using
> > mapping->writepage. This can result in very deep call stacks, particularly
> > if the target storage or filesystem are complex. It has already been observed
> > on XFS that the stack overflows but the problem is not XFS-specific.
> >
> > This patch prevents direct reclaim writing back filesystem pages by checking
> > if current is kswapd or the page is anonymous before writing back. If the
> > dirty pages cannot be written back, they are placed back on the LRU lists
> > for either background writing by the BDI threads or kswapd. If in direct
> > lumpy reclaim and dirty pages are encountered, the process will stall for
> > the background flusher before trying to reclaim the pages again.
> >
> > As the call-chain for writing anonymous pages is not expected to be deep
> > and they are not cleaned by flusher threads, anonymous pages are still
> > written back in direct reclaim.
> >
> > Signed-off-by: Mel Gorman <mel@....ul.ie>
> > Acked-by: Rik van Riel <riel@...hat.com>
> > Reviewed-by: Johannes Weiner <hannes@...xchg.org>
> > ---
> > mm/vmscan.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++---
> > 1 files changed, 46 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > index ff52b46..408c101 100644
> > --- a/mm/vmscan.c
> > +++ b/mm/vmscan.c
> > @@ -145,6 +145,9 @@ static DECLARE_RWSEM(shrinker_rwsem);
> > #define scanning_global_lru(sc) (1)
> > #endif
> >
> > +/* Direct lumpy reclaim waits up to five seconds for background cleaning */
> > +#define MAX_SWAP_CLEAN_WAIT 50
> > +
> > static struct zone_reclaim_stat *get_reclaim_stat(struct zone *zone,
> > struct scan_control *sc)
> > {
> > @@ -682,11 +685,13 @@ static noinline_for_stack void free_page_list(struct list_head *free_pages)
> > * shrink_page_list() returns the number of reclaimed pages
> > */
> > static unsigned long shrink_page_list(struct list_head *page_list,
> > - struct scan_control *sc)
> > + struct scan_control *sc,
> > + unsigned long *nr_still_dirty)
> > {
> > LIST_HEAD(ret_pages);
> > LIST_HEAD(free_pages);
> > int pgactivate = 0;
> > + unsigned long nr_dirty = 0;
> > unsigned long nr_reclaimed = 0;
> >
> > cond_resched();
> > @@ -785,6 +790,15 @@ static unsigned long shrink_page_list(struct list_head *page_list,
> > }
> >
> > if (PageDirty(page)) {
> > + /*
> > + * Only kswapd can writeback filesystem pages to
> > + * avoid risk of stack overflow
> > + */
> > + if (page_is_file_cache(page) && !current_is_kswapd()) {
> > + nr_dirty++;
> > + goto keep_locked;
> > + }
> > +
> > if (references == PAGEREF_RECLAIM_CLEAN)
> > goto keep_locked;
> > if (!may_enter_fs)
> > @@ -908,6 +922,8 @@ keep_lumpy:
> > free_page_list(&free_pages);
> >
> > list_splice(&ret_pages, page_list);
> > +
> > + *nr_still_dirty = nr_dirty;
> > count_vm_events(PGACTIVATE, pgactivate);
> > return nr_reclaimed;
> > }
> > @@ -1312,6 +1328,10 @@ static inline bool should_reclaim_stall(unsigned long nr_taken,
> > if (sc->lumpy_reclaim_mode == LUMPY_MODE_NONE)
> > return false;
> >
> > + /* If we cannot writeback, there is no point stalling */
> > + if (!sc->may_writepage)
> > + return false;
> > +
> > /* If we have relaimed everything on the isolated list, no stall */
> > if (nr_freed == nr_taken)
> > return false;
> > @@ -1339,11 +1359,13 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
> > struct scan_control *sc, int priority, int file)
> > {
> > LIST_HEAD(page_list);
> > + LIST_HEAD(putback_list);
> > unsigned long nr_scanned;
> > unsigned long nr_reclaimed = 0;
> > unsigned long nr_taken;
> > unsigned long nr_anon;
> > unsigned long nr_file;
> > + unsigned long nr_dirty;
> >
> > while (unlikely(too_many_isolated(zone, file, sc))) {
> > congestion_wait(BLK_RW_ASYNC, HZ/10);
> > @@ -1392,14 +1414,35 @@ shrink_inactive_list(unsigned long nr_to_scan, struct zone *zone,
> >
> > spin_unlock_irq(&zone->lru_lock);
> >
> > - nr_reclaimed = shrink_page_list(&page_list, sc);
> > + nr_reclaimed = shrink_page_list(&page_list, sc, &nr_dirty);
> >
> > /* Check if we should syncronously wait for writeback */
> > if (should_reclaim_stall(nr_taken, nr_reclaimed, priority, sc)) {
>
> It is possible to OOM if the LRU list is small and/or the storage is slow, so
> that the flusher cannot clean enough pages before the LRU is fully scanned.
>
To go OOM, nr_reclaimed would have to be 0 and for that, the entire list
would have to be dirty or unreclaimable. If that situation happens, is
the dirty throttling not also broken?
> So we may need do waits on dirty/writeback pages on *order-0*
> direct reclaims, when priority goes rather low (such as < 3).
>
In case this is really necessary, the necessary stalling could be done by
removing the check for lumpy reclaim in should_reclaim_stall(). What do
you think of the following replacement?
/*
* Returns true if the caller should wait to clean dirty/writeback pages.
*
* If we are direct reclaiming for contiguous pages and we do not reclaim
* everything in the list, try again and wait for writeback IO to complete.
* This will stall high-order allocations noticeably. Only do that when really
* need to free the pages under high memory pressure.
*
* Alternatively, if priority is getting high, it may be because there are
* too many dirty pages on the LRU. Rather than returning nr_reclaimed == 0
* and potentially causing an OOM, we stall on writeback.
*/
static inline bool should_reclaim_stall(unsigned long nr_taken,
unsigned long nr_freed,
int priority,
struct scan_control *sc)
{
int stall_priority;
/* kswapd should not stall on sync IO */
if (current_is_kswapd())
return false;
/* If we cannot writeback, there is no point stalling */
if (!sc->may_writepage)
return false;
/* If we have relaimed everything on the isolated list, no stall */
if (nr_freed == nr_taken)
return false;
/*
* For high-order allocations, there are two stall thresholds.
* High-cost allocations stall immediately where as lower
* order allocations such as stacks require the scanning
* priority to be much higher before stalling.
*/
if (sc->order > PAGE_ALLOC_COSTLY_ORDER)
stall_priority = DEF_PRIORITY;
else
stall_priority = DEF_PRIORITY / 3;
return priority <= stall_priority;
}
> > + int dirty_retry = MAX_SWAP_CLEAN_WAIT;
> > set_lumpy_reclaim_mode(priority, sc, true);
> > - nr_reclaimed += shrink_page_list(&page_list, sc);
> > +
> > + while (nr_reclaimed < nr_taken && nr_dirty && dirty_retry--) {
> > + struct page *page, *tmp;
> > +
>
> > + /* Take off the clean pages marked for activation */
> > + list_for_each_entry_safe(page, tmp, &page_list, lru) {
> > + if (PageDirty(page) || PageWriteback(page))
> > + continue;
> > +
> > + list_del(&page->lru);
> > + list_add(&page->lru, &putback_list);
> > + }
>
> nitpick: I guess the above loop is optional code to avoid overheads
> of shrink_page_list() repeatedly going through some unfreeable pages?
Pretty much, if they are to be activated, there is no point trying to reclaim
them again. It's unnecessary overhead. A strong motivation for this
series is to reduce overheads in the reclaim paths and unnecessary
retrying of unfreeable pages.
> Considering this is the slow code path, I'd prefer to keep the code
> simple than to do such optimizations.
>
> > + wakeup_flusher_threads(laptop_mode ? 0 : nr_dirty);
>
> how about
> if (!laptop_mode)
> wakeup_flusher_threads(nr_dirty);
>
It's not the same thing. wakeup_flusher_threads(0) in laptop_mode is to
clean all pages if some need dirtying. laptop_mode cleans all pages to
minimise disk spinups.
> > + wait_iff_congested(zone, BLK_RW_ASYNC, HZ/10);
> > +
> > + nr_reclaimed = shrink_page_list(&page_list, sc,
> > + &nr_dirty);
> > + }
> > }
> >
> > + list_splice(&putback_list, &page_list);
> > +
> > local_irq_disable();
> > if (current_is_kswapd())
> > __count_vm_events(KSWAPD_STEAL, nr_reclaimed);
> > --
> > 1.7.1
>
--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab
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